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38 SMT007 MAGAZINE I FEBRUARY 2020 drite growth-related field returns. When pro- cessing with no-clean flux, the focus is on the thermal profile for ensuring that all flux acti- vators are properly complexed with the activa- tors outgassed as designed by the flux manu- facturers. Conversely, when an aqueous wash process is used after assembly, it is important to be sure the process is fully removing all of the residues. If you are washing a water-solu- ble flux, any remaining residue will be conduc- tive without question, and if washing no-clean flux, any remaining residue may or may not be conductive. Washing a no-clean flux is especially chal- lenging when parts with tight pitch spacing are used because the outer shell of the flux can effectively create a dam between the leads and prohibit the wash solution from penetrating under the part as necessary to remove all the residues. Far too often, we see customers that had been using a water-soluble flux migrate over to a no-clean flux and use the same recipe that was previously effective without under- standing the differences in the materials. The problem with that is the extra energy required to break through the outer resin shell that is created through the thermal excursion that is meant to help prohibit moisture from reaching any active residues. If the process is only breaking down that outer shell, it will often leave behind the resi- dues that were meant to be protected and can be just as conductive as water-soluble flux. This is why testing at the specific fine pitch component locations with ion chromatogra- phy is so important whether you are cleaning or not. IC testing will yield data that tells you the type and amount of ionic content present. That tells you if you are at an increased risk of electrical leakage in the field. This all speaks to your product's reliability, which, of course, speaks to your company's bottom line and reputation. Cleanliness is one of the most important aspects to consider when using these types of components but certainly not the only aspect. Another thing to consider is the paste print- ing process. When you print solder paste for these parts, there is an increased risk of bridg- ing after reflow soldering. This can increase hand rework after reflow, and every time you subject your product to a human, you increase the risk of damage. When a machine is making a mistake, it will normally make that mistake repeatedly, and those types of issues are fairly easy to discover and fix. When you send your assemblies to one of many operators in the rework area, the risk of one-off damage increases because it is much harder to control each operator's pro- cess. There may even be a need to evaluate the paste flux currently being used to deter- mine if that is still a viable material for your process. All major flux manufacturers offer a range of options that specially address fine- pitch components, so if you need to make a change, there should be no shortage of mate- rials available. The bottom line, from my point of view regarding reliability on miniaturization, is the same as with any assembly process; you have to do the required chemical and environmental testing to determine if your assembly parame- ters are properly processing the chosen mate- rial set in a way that does not increase the risk of electrical leakage in the field. Now, we just need to work on a popcorn popper that fits in my other pocket. SMT007 Eric Camden is a lead investigator at Foresite Inc. To read past columns or contact Camden, click here. The reduction in component pitch puts a premium on cleanliness to reduce the risk of dendrite growth- related field returns.

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